feat: Add comprehensive plugin documentation and advanced template system

Added complete plugin documentation with visual mockups and expanded template
variable system with CONTEXT, DATE helpers, and default values.

Backend Changes:
- Extended template_parser.py to support all new template types
- Added PROMPT with default values: {{PROMPT:var|desc|default}}
- Added CONTEXT variables: {{CONTEXT:business_name}}, {{CONTEXT:owner_email}}
- Added DATE helpers: {{DATE:today}}, {{DATE:+7d}}, {{DATE:monday}}
- Implemented date expression evaluation for relative dates
- Updated compile_template to handle all template types
- Added context parameter for business data auto-fill

Frontend Changes:
- Created comprehensive HelpPluginDocs.tsx with Stripe-style API docs
- Added visual mockup of plugin configuration form
- Documented all template types with examples and benefits
- Added Command Reference section with allowed/blocked Python commands
- Documented all HTTP methods (GET, POST, PUT, PATCH, DELETE)
- Added URL whitelisting requirements and approval process
- Created Platform Staff management page with edit modal
- Added can_approve_plugins and can_whitelist_urls permissions

Platform Staff Features:
- List all platform_manager and platform_support users
- Edit user details with role-based permissions
- Superusers can edit anyone
- Platform managers can only edit platform_support users
- Permission cascade: users can only grant permissions they have
- Real-time updates via React Query cache invalidation

Documentation Highlights:
- 4 template types: PROMPT, CONTEXT, DATE, and automatic validation
- Visual form mockup showing exactly what users see
- All allowed control flow (if/elif/else, for, while, try/except, etc.)
- All allowed built-in functions (len, range, min, max, etc.)
- All blocked operations (import, exec, eval, class/function defs)
- Complete HTTP API reference with examples
- URL whitelisting process: contact pluginaccess@smoothschedule.com

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

frontend/src/App.tsx

@@ -50,6 +50,7 @@ import PlatformDashboard from './pages/platform/PlatformDashboard';
 import PlatformBusinesses from './pages/platform/PlatformBusinesses';
 import PlatformSupportPage from './pages/platform/PlatformSupport';
 import PlatformUsers from './pages/platform/PlatformUsers';
+import PlatformStaff from './pages/platform/PlatformStaff';
 import PlatformSettings from './pages/platform/PlatformSettings';
 import ProfileSettings from './pages/ProfileSettings';
 import VerifyEmail from './pages/VerifyEmail';
@@ -60,6 +61,7 @@ import Tickets from './pages/Tickets'; // Import Tickets page
 import HelpGuide from './pages/HelpGuide'; // Import Platform Guide page
 import HelpTicketing from './pages/HelpTicketing'; // Import Help page for ticketing
 import HelpApiDocs from './pages/HelpApiDocs'; // Import API documentation page
+import HelpPluginDocs from './pages/HelpPluginDocs'; // Import Plugin documentation page
 import PlatformSupport from './pages/PlatformSupport'; // Import Platform Support page (for businesses to contact SmoothSchedule)
 import { Toaster } from 'react-hot-toast'; // Import Toaster for notifications
 
@@ -324,12 +326,14 @@ const AppContent: React.FC = () => {
               <Route path="/platform/dashboard" element={<PlatformDashboard />} />
               <Route path="/platform/businesses" element={<PlatformBusinesses onMasquerade={handleMasquerade} />} />
               <Route path="/platform/users" element={<PlatformUsers onMasquerade={handleMasquerade} />} />
+              <Route path="/platform/staff" element={<PlatformStaff />} />
             </>
           )}
           <Route path="/platform/support" element={<PlatformSupportPage />} />
           <Route path="/help/guide" element={<HelpGuide />} />
           <Route path="/help/ticketing" element={<HelpTicketing />} />
           <Route path="/help/api" element={<HelpApiDocs />} />
+          <Route path="/help/plugins" element={<HelpPluginDocs />} />
           {user.role === 'superuser' && (
             <Route path="/platform/settings" element={<PlatformSettings />} />
           )}
@@ -509,6 +513,7 @@ const AppContent: React.FC = () => {
           <Route path="/help/guide" element={<HelpGuide />} />
           <Route path="/help/ticketing" element={<HelpTicketing />} />
           <Route path="/help/api" element={<HelpApiDocs />} />
+          <Route path="/help/plugins" element={<HelpPluginDocs />} />
           <Route path="/support" element={<PlatformSupport />} />
           <Route
             path="/customers"